Definition of the antenna temperature

 

Let us assume now that our telescope observes over a limited frequency range, , (the so-called "passband") with constant response over that range. When pointing at a blank portion of the sky, there are only contributions from what one would call "background" or "noise". The power of this component can be expressed as


 

 

Similarly, the power of an astronomical source can be expressed as


 

 

where


 

 

is the so-called "antenna temperature", which is a measure of the temperature equivalent of the power received from an astronomical source.

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